Conventionally, for improving the buffer hardness for the sitter's waist and spinal column, the back-rest of the chair on board an aircraft or bus has been installed with an adjustable waist supporter to lessen the fatigue. An example is disclosed in Japanese Laid Open Pat. No. 57-826.
Although office chairs also have the need of being equipped with an adjustable waist supporter, it is very difficult in act to apply the same adjusting mechanism as used on board an aircraft or bus to the office chair, owing to the former being operated by a mechanism installed on the chair's side to made the back-rest move forward or backward using a complex lever and gear construction.